Etymology
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signal (n.)
late 14c., "visible sign, indication," from Old French signal, seignal "seal, imprint, sign, mark," from Medieval Latin signale "a signal," from Late Latin signalis (adj.) "used as a signal, pertaining to a sign," from Latin signum "identifying mark, sign" (see sign (n.)). Restricted sense "agreed-upon sign (to commence or desist, etc.) is from 1590s. Meaning "modulation of an electric current" is from 1855.
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prompt (adj.)

early 15c., "ready, prepared (to do something), quick to act as occasion demands," from Old French prompt (13c.) and directly from Latin promptus "brought forth," hence "visible, apparent, evident, at hand," past-participle adjective from promere "to take or bring out or forth" (see prompt (v.)). Meaning "given or performed without delay" is from 1520s. Related: Promptly.

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Pleiades (n.)

late 14c., Pliades, "visible open star cluster in the constellation Taurus," in Greek mythology they represent the seven daughters of Atlas and Pleione, transformed by Zeus into seven stars. From Latin Pleiades, from Greek Pleiades (singular Plēias), perhaps literally "constellation of the doves" from a shortened form of peleiades, plural of peleias "dove" (from PIE root *pel- "dark-colored, gray"). Or perhaps from plein "to sail," because the season of navigation begins with their heliacal rising.

Old English had the name from Latin as Pliade. The star cluster is mentioned by Hesiod (pre-700 B.C.E.); only six now are visible to most people; on a clear night a good eye can see nine (in 1579, well before the invention of the telescope, the German astronomer Michael Moestlin (1550-1631) correctly drew 11 Pleiades stars); telescopes reveal at least 500. Hence French pleiade, used for a meeting or grouping of seven persons.

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disappear (v.)

early 15c., disaperen, "cease to be visible, vanish from sight, be no longer seen," from dis- "do the opposite of" + appear. Earlier was disparish (early 15c.), from French disparaiss-, stem of desapparoistre (Modern French disparaître).

Transitive sense, "cause to disappear," is from 1897 in chemistry; by 1948 of inconvenient persons. Related: Disappeared; disappearing; disappears. Slang disappearing act "fact of absconding, action of getting away," is attested by 1884, probably originally a reference to magic shows.

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blue-blood (adj.)
1809 in reference to the blood that flows in the veins of the old and aristocratic families of Spain, translating Spanish sangre azul, claimed by certain families of Castile that held themselves uncontaminated by Moorish or Jewish admixture; the term probably is from the notion of the visible veins of people of fair complexion. In reference to English families by 1827. As a noun, "member of an old and aristocratic family," by 1877. See blue (adj.1) + blood (n.).
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Procyon (n.)

bright star in the constellation Canis Minor (the 8th brightest in the sky), 1650s, from Latin, from Greek Prokyōn, the name of the star or the constellation (which has few other visible stars), from pro "before" (see pro-) + kyōn "dog" (from PIE root *kwon- "dog"). So called from its rising just before the "Dog Star," Sirius.

By Roman astronomers, sometimes Latinized as Antecanis. A mid-15c. English Prochion seems to refer to the constellation.

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existence (n.)
late 14c., "reality," from Old French existence, from Medieval Latin existentia/exsistentia, from existentem/exsistentem (nominative existens/exsistens) "existent," present participle of Latin existere/exsistere "stand forth, come out, emerge; appear, be visible, come to light; arise, be produced; turn into," and, as a secondary meaning, "exist, be;" from ex "forth" (see ex-) + sistere "cause to stand," from PIE *si-st-, reduplicated form of root *sta- "to stand, make or be firm."
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figure (n.)

c. 1200, "numeral;" mid-13c., "visible appearance of a person;" late 14c., "visible and tangible form of anything," from Old French figure "shape, body; form of a word; figure of speech; symbol, allegory" (10c), from Latin figura "a shape, form, figure; quality, kind, style; figure of speech," in Late Latin "a sketch, drawing" (from PIE root *dheigh- "to form, build").

Philosophical and scientific senses are from use of Latin figura to translate Greek skhema. Meaning "lines forming a shape" is from mid-14c. From mid-14c. as "human body as represented by art;" late 15c. as "a body, the human form as a whole."  From late 14c. as "a cut or diagram inserted in text."

The rhetorical use of figure, "peculiar use of words giving meaning different from usual," dates to late 14c.; hence figure of speech (1550s). Figure-skating is from 1835, so called for the circular patterns skaters formerly made on the ice to demonstrate control; they were dropped from international competition in 1990, but the name remains. Figure eight as a shape was originally figure of eight (c. 1600).

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appear (v.)

late 13c., "come into view," from stem of Old French aparoir, aperer "appear, come to light, come forth" (12c., Modern French apparoir), from Latin apparere "to appear, come in sight, make an appearance," from ad "to" (see ad-) + parere "to come forth, be visible; submit, obey," which is of uncertain origin; de Vaan says from a PIE *prh-o- "providing." Of persons, "present oneself," late 14c. Meaning "seem, have a certain appearance" is late 14c. Related: Appeared; appearing.

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optic (adj.)

late 14c., optik, "of or pertaining to the eye as the organ of vision," from Old French optique, obtique (c. 1300) and directly from Medieval Latin opticus "of sight or seeing," from Greek optikos "of or having to do with sight," from optos "seen, visible," related to ōps "eye," from PIE root *okw- "to see." Meaning "relating to or pertaining to vision or sight" is from 1590s. Optics "eyes" is from 1640s; "formerly the learned and elegant term; afterwards pedantic, and now usually humorous" [OED].

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